fix(ui): a slider gutter you can actually see, and no Debug tab in the show UI

[pyramation]

Summary

The slider gutter was invisible regardless of colour, and the fix in #122 couldn't have worked. The track was painted on the input itself:

input[type="range"] {
  height: 10px;          /* border-box, per Tailwind preflight */
  padding: 8px 0;        /* 16px > 10px, so the content box is 0px tall */
  background: linear-gradient(...);
  background-clip: content-box;   /* clips to a zero-height box: nothing */
}

Any slider whose height was under the 16px of padding — the default, and the inline height: 24/28 overrides — rendered no track at all. Painting the gutter on ::-webkit-slider-runnable-track / ::-moz-range-track instead decouples the visible channel from the element's height, so the element height is purely the touch target and components can override it freely. The thumb gets margin-top: -11px to stay centred on the thinner track (-12px at the coarse-pointer size). Colour is #1c2b4d — the panel stays black, only the channel lifts off it.

Also drops the debug tab from the show UI (debugging lives in the desktop shell now), which leaves SettingsTab unreferenced, so it's deleted along with the 'debug' member of GridMode and the tab-change calibration teardown that existed only for it. Tab strip padding and pill padding come in a couple of px.

Link to Devin session: https://app.devin.ai/sessions/ec43152136134467a853ec0bbf783ea9
Requested by: @pyramation

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